1. Technical Field
The present disclosure relates to a pixel circuit, a driving method thereof, and an image sensor, in particular, to a bipolar junction transistor (BJT) pixel circuit, a driving method thereof, and an image sensor.
2. Description of Related Art
An optical mouse, typically includes a light source to illuminate a navigation surface and an image sensor that functions as a miniature digital camera to continually capture images of the illuminated navigation surface and to determine the speed and direction that the optical mouse is being moved across the surface by comparing sequentially captured frames of image information so as to control the movement of a cursor displayed on a monitor. Optical mouse is generally equipped with a battery (e.g., dry cell battery) for supplying the necessary operating power.
Bipolar junction transistor (BJT) pixel circuit is one of the pixel circuits used in an image sensor of the optical mouse for capturing image data of the illuminated navigation surface. Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 shows a schematic diagram illustrating a conventional BJT pixel circuit. FIG. 2 shows a timing waveform diagram illustrating the operation of the conventional BJT pixel circuit.
The BJT pixel circuit 9 includes a feedback amplifier circuit 91, a BJT transistor 93, a photodiode 95, a shutter circuit 97, and a storage capacitor 99. The feedback amplifier circuit 91, the photodiode 95, and the shutter circuit 97 are coupled to the BJT transistor 93, respectively. The storage capacitor 99 is coupled to the shutter circuit 97 and a ground GND, wherein the storage capacitor 99 is used for storing an image data captured by the photodiode 95.
The operation of the BJT pixel circuit 9 can be briefly described as follow. At start of each frame (e.g., time point TA), a light source (not shown) is pulsed on with a light control signal LS to generate a reflect light shone on the photodiode 95. The photodiode 95 operatively generates a base current IB responsive to the light intensity of a light illuminated from a light source. From time point TA and time point TB, the feedback amplifier circuit 91 begins to build up voltage across the emitter and the base of the BJT transistor 93 to increase the beta of the BJT transistor 93 to a stable level e.g., ˜30, wherein the beta herein is the ratio between the collector current IC and the base current IB of the BJT transistor 93. Next, at time point TB, the BJT transistor 93 operates with a stable beta and generates a stable emitter current IE. The shutter circuit 97 is triggered on at the same time by a shutter signal such that the storage capacitor 99 can be discharged with a stable emitter current IE to capture the image data. Accordingly, when the shutter circuit 97 is triggered on, the voltage VSC across the storage capacitor 99 decreases as the storage capacitor 99 discharges through BJT transistor 93 with the emitter current IE.
Referring to FIG. 2 again, the time required from the light source turning on (e.g., time point TA) to the time the BJT transistor 93 obtaining a stable beta (e.g., time point TB) is defined as the pre-flash time of the BJT pixel circuit 9. The time required from the shutter circuit being triggered on (e.g., time point TB) until the completion of image capturing is defined as the exposure time. Particularly, for the BJT pixel circuit 9 provided in FIG. 1, the pre-flash time required is around 220 us while the time required for the shutter circuit 97 to be triggered on is around 10 s. Hence, the total duration that the light source needed to be turned on is approximately 230 us.
It is commonly known that any image captured with insufficient pre-flash time will appear to be darker due to low beta level. However, the amount of duration of the pre-flash time of the BJT transistor 93 and the on-time duration of light source has directly impact on the power consumption of the image sensor. That is, the longer the pre-flash time required by the BJT transistor 93, the longer the light source needed to be turned on, and more power is consumed by the optical mouse thereby reduce the battery life.